The present invention relates to a tray to be used when sintering powder metallurgical articles such as products of cemented carbides and cermets.
Cemented carbides and cermets are alloys mainly containing hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder essentially based on Co and/or Ni and/or Fe. They are normally produced by powder metallurgical methods comprising milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering.
After pressing, the compacted bodies contain about 50% by volume of open porosity. Fully dense products are then produced by liquid phase sintering at a temperature when the binder metal is in liquid state, which in practice is normally in the 1300-1550.degree. C. range depending on composition.
In the production of cemented carbides and cermets, almost all sintering processes are performed in an industrial vacuum, i.e., from about 1 torr upwards and in the presence of various gas atmospheres, such as mixtures of H.sub.2 plus CO, CO.sub.2, Ar, N.sub.2, CH.sub.4 etc., and with the bodies to be sintered lying on graphite trays. In order to minimize the influence of the graphite, both in the contact with the bodies to be sintered and with the atmosphere within the furnace, these trays are normally surface coated, see e.g., H. Kolaska et al., "Powder Metallurgy of Hardmetals", EPMA Lecture Series, Lecture 6, Sintering--Technical and Basic Principles, European Powder Metallurgy Association, Shrewsbury, UK, 1995, p.6/9-6/10. In practice, an A1.sub.2 O.sub.3 or ZrO.sub.2 based coating is used, applied by thermal spraying, preferably plasma powder spraying. Thus, these coatings act as barrier layers preventing reaction from occurring between the sintered body and the tray. Reaction can lead to many problems, i.e., carbon uptake from the trays and distortion of the sintered bodies.
It is essential that these graphite trays with barrier layers be reused for as many sintering cycles as possible without deterioration of the layers occurring, i.e., to maintain inertness and high adhesion to the graphite tray. Normally, the necessity to regrind and recoat the trays is determined by the skilled operator when the coating has been torn off to such an extent that it is easily seen by the naked eye of the operator.
The deterioration is due to metallurgical reactions taking place between the cemented carbide body and the tray leading to unloading problems caused by sticking. When removing such bodies adhering to the tray, cracks may be formed in the coating of the tray. Individual grains from the very surface of the coating or pieces of the coating may be torn off. The problem is especially severe when sintering large bodies of cemented carbides having comparatively large contents of binder phase, these conditions promoting adhesion.